Chicago police said a lifeguard saw someone go underwater around 7 p.m. Sunday night and did not resurface at Foster Beach.
Several crews responded, including the Marine Unit, CFD Divers and the helicopter to search for the missing male person.
Police said the beach, near the city's Edgewater neighborhood, was very crowded at the time.
Crews said they searched extensively, but nothing was found.
Police say it's unclear if and when they will resume the search Monday morning.